This invention relates to aluminosilicate zeolites, and more particularly to the synthesis and application of aluminum rich (low silica) AFI and GME zeolites.
Zeolites are molecular sieves that have a silicate lattice. Typically, they are microporous crystalline materials that can have a variable composition and are characterized by a three dimensional structure that has channels and/or cages. Silicon (SiO4) or aluminum (AlO4) tetrahedrons makeup the zeolite framework.
Zeolites are differentiated from each other by their composition and structure, which determines their physical and chemical properties and the applications in which they will be useful. Typically, structure codes consisting of three letters are assinged to each zeolite. For example, FAU is the structure code for zeolite X, GME for gmelinite, and AFI for AlPO4-5 molecular sieve.
Many zeolites may be reversibly dehydrated with only minor distortions of the framework. It has been shown that the thermal stability of zeolites depends on the cation form. For example, the Na forms of CHA-(chabazite) and EAB-TMA-E(AB) type zeolites (having 6-ring sequences of AABBCC and ABBACC, respectively) transform topotactically to SOD type (sodalite) products above 600xc2x0 C. in dry N2. The temperature of this transformation depends greatly on the Al content, the numbers of protons, and the amount of water present. By breaking two Txe2x80x2-O-T bridges in a catalytic reaction with water, pivoting Txe2x80x2 about the remaining Txe2x80x2-O-T bridges leads to inversion of Txe2x80x2O4 tetrahedra, where Txe2x80x2 is Si or Al and T is Si or Al. One noteworthy observation is that potassium ions in 8-ring sites of K exchanged CHA and EAB prevent their transformation to SOD products. In contrast, Na exchanged CHA and EAB transform to SOD products.
Gmelinite (GME) is a well-known aluminosilicate zeolite having a structure in which the main feature is a large 12-ring channel. GME exists naturally as a mineral, and can also be synthesized in the laboratory. CHA-free gmelinite can be synthesized using a polymeric template synthesis system. In addition, another method to synthesize large pore gmelinite is to introduce a transition metal, such as Cr, into the gmelinite framework.
Gmelinite zeolites have a large-pore structure with channels that are defined by twelve membered rings of SiO4 and AlO4. However, the adsorptive properties of gmelinite zeolites are similar to zeolites having smaller pores. The reason for this is that natural and synthetic gmelinites have a propensity to intergrow with chabazite or related zeolites, which creates stacking faults that block and restrict access to the 12-ring channel of the gmelinite structure. The result is a reduction in the expected sorption properties of the zeolite. Elimination of these fault planes would likely increase the adsorptive properties of the gmelinite zeolite. To be a good adsorbent, a zeolite should have a high degree of crystallinity both in its synthesized or natural form and active form.
The AFI zeolite also has a 12-ring structure with large pores. These aluminosilicate materials, particularly aluminum rich (low silica) materials, have been used to separate and purify gases, exchange ions, catalytically convert inorganic/organic compounds, and serve as catalyst supports. The structure comprises a one-dimensional 7.5 A diameter pore system constrained by 12-rings, with relatively smooth channels devoid of cavities. Chevron Research Company has synthesized an all-silica AFI material, known as SSZ-24, using special templates, such as N,N,N-trimethyl-1-adamantammonium hydroxide. Boron-SSZ-24 has also been made by direct synthesis.
There are some similarities between GME and AFI zeolite structures. In the GME framework, tetrahedra are pointing up (U) and down (D) and are commonly described in terms of UUDD chains of 4-rings. For example, UUDD chains are found in the zeolite framework of philipsite, gismondine, gmelinite, and merlinoite.
On the other hand, the AFI framework, has tetrahedra pointing up, with adjacent units pointing down. The AFI framework can be described as UDUD. UDUD chains are found in AlPO-5, AlPO-11, AlPO-25 and AlPO-D.
Most UUDD chains occur in silicate materials, whereas most UDUD chains are found in aluminophosphate materials. This may explain why SSZ-24 does not form as readily as does AlPO-5 because O atoms are connected to the 4-rings in the UDUD chains. In the AlPO-5 structure, one of the Alxe2x80x94Oxe2x80x94P angles was recorded to be 178xc2x0. This appears to be undesirable in silicate frameworks.
The aluminum ions in the zeolite framework creates an excess negative charge, which can be balanced by ions of alkaline metals (Na, K, Li, Rb and Cs), alkaline earth metals (Mg, Ca, Ba), organic ammonium cations, or hydrogen ions (H+). High aluminum content can increase zeolite acidity, requiring more cations to balance the zeolite framework""s negative charges. This enhances the zeolite""s catalytic properties.
The focus of the prior art has primarily been to increase the quality of the gmelinite zeolite. Various templates, reaction compositions, and conditions have been tested. Several patents describe the processes employed to increase the quality of the gmelinite zeolite, although none disclose the method of the present invention:
U.S. Pat. No. 4,061,717 to Kerr et al. discloses a process for synthesizing crystalline aluminosilicate zeolites, which are crystallized in an aqueous reaction mixture containing sources of alkali metal oxide, silicate, aluminate and an ionene or ionomer which contains positively charged nitrogen atoms in such quantity as to satisfy some of the cationic sites of the eventual zeolite. In the patent, Kerr described the synthesis of gmelinite using 1,4-dibromobutane (Dab-4Br) as a template. The template serves to eliminate or decrease the number of stacking faults, which normally appear to block gmelinite channels.
U.S. Pat. No. 5,283,047 to Vaughan et al. teaches a synthetic transition metal aluminosilicate crystalline zeolite having a gmelinite structure, a defined chemical composition that has been characterized by a defined X-ray diffraction pattern.
U.S. Pat. No. 4,665,110 to Zones teaches the making of crystalline molecular sieves, such as zeolites, using adamantane compounds as templates. One of the zeolites that may be synthesized is an AFI zeolite known as SSZ-24, which has the AFI structure.
Zones later made Al-SSZ-24 by post-synthesis treating B-SSZ-24 (R. A. van Nordstrand, D. S. Santilli, S. I. Zones, xe2x80x9cAluminum- and Boron-Containing SSZ-24xe2x80x9d, in Synthesis of Microporous Materials, Vol. 1, Molecular sieves (eds. M. L. Occelli, H. Robson), 1992, p.373. Van Nostrand Reinhold, New York). The resulting Al-SSZ-24 has a low aluminum content with a ratio of SiO2/Al2O3=100.
SSZ-24 has been used in catalytic applications and separations such as: (1) reforming naphtha with SSZ-24; (2) catalytic reforming naphtha with boron-SSZ-24; (3) extraction of dimethyl paraffins from isomerates relating to the production of high octane fuels with SSZ-24; (4) materials (SSZ-24, Al-SSZ-24, B-SSZ-24, AlPO-5 and SAPO-5) with AFI structure have a strong affinity for the adsorption of branched chain hexanes; (5) materials (Al-SSZ-24 and B-SSZ-24) have inverse shape selectivity as a catalyst.
However, the catalysis, adsorption, and ion-exchange utility of SSZ-24 is limited by the extremely low aluminum content available from the prior art synthesis procedure. This is a significant drawback and hindrance.
Cartlidge (S. Cartlidge, W. M. Meier, Zeolites, 1984, 4, 218 and S. Cartlidge, E. B. Keller, W. M. Meier, Zeolites, 1984, 4, 226) explained how Na-EAB (zeolite) transitioned into SOD and the stabilization role of potassium ions in K-EAB.
However, the prior art does not teach how to prepare high purity low silica AFI zeolite for potential use in adsorption and catalytic applications. Furthermore, the prior art does not teach how to remove a template from a high purity (fault-free and intergrowth-free) low silica gmelinite zeolite (GME) without changing the GME structure.
It is an object of the invention to synthesize low silica gmelinite zeolite adsorbents with high N2/O2 selectivity.
It is another object of the invention to synthesize low silica AFI zeolite catalysts.
It is yet another object of the invention to synthesize low silica AFI zeolite adsorbents.
It is still yet another object of the invention to synthesize high purity low silica gmelinite zeolites.
It is still yet another object of the invention to provide a high purity gmelinite zeolite that has a SiO2/Al2O3 ratio that is about 10 or less.
It is another object of the invention to prepare a gmelinite zeolite that does not have a template.
These and other objects are achieved by the present invention, which includes an AFI zeolite that has a SiO2/Al2O3 ratio of about 10 or less.
The present invention also includes a high purity gmelinite zeolite that has a SiO2/Al203 ratio of about 10 or less.